1. Field of the Invention
The present invention relates to a magnetoresistance effect device, more particularly, relates to a magnetoresistance effect device used for a magnetic head, memory or the like, which is capable of making a magnetostrictive constant small with a MR ratio maintained to be high.
2. Description of the Related Art
A magnetoresistance effect device has been used for MRAM (Magnetic Random Access Memory) known as a nonvolatile memory, a magnetic head, a magnetic sensor or the like. The magnetoresistance effect device has fundamentally a film structure including a three-layer structure of a fixed ferromagnetism layer/a barrier layer/a free ferromagnetism layer. When passing an electric current between the two ferromagnetic layers in the magnetoresistance effect device, an electric resistance value of the device becomes a small value (RP) if the directions of magnetization of the two ferromagnetic layers are parallel to a layer direction and the same, while the electric resistance value becomes a large value (RA) if the above directions are parallel to the layer direction and opposite. The size in change of the electric resistance dependent on the direction of magnetization of the two ferromagnetic layers is indicated by a MR ratio, and this MR ratio is defined as a following formula.MR ratio(%)=(RA−RP)×100÷RP 
In the MRAM, reading information is performed on the basis of a change of the electric resistance by the difference in the direction of magnetization of the two ferromagnetic layers in a TMR device. Therefore, the higher the MR ratio becomes, the larger is an output voltage of the MRAM.
Concerning the magnetoresistance effect device mentioned above, the inventors of the present invention have discovered a device with a very high MR ratio of 230%. This magnetoresistance effect device had a characteristic that amorphous CoFeB was used as the free ferromagnetism layer, a Magnesium Oxide (MgO) film was grown up for the barrier layer, and further film structure including the three-layers structure of CoFeB (fixed ferromagnetism layer)/MgO (middle layer)/CoFeB (free ferromagnetism layer). The MgO film had single crystal structure or high orientation polycrystal structure (fiber texture) without grain boundary in a thickness direction. In accordance with the magnetoresistance with the above characteristic structure, the MR ratio of 230% has been obtained. This fact was disclosed in the reference of “the Applied Physics Letters on Feb. 23, 2005 on-line issue”, as the article of “230% room-temperature magnetoresistance in CoFeB/MgO/CoFeB magnetic tunnel junction” written by David D Djayaprawira, etc.
In the devices such as a magnetic head, a memory, a magnetic sensor or the like, there is a magnetostrictive constant as one of amounts of measurement to be managed for a good device operation. When setting a film stress as σ, a magnetostrictive constant as λ, and saturation magnetization as Ms concerning magnetic material, an anisotropy magnetic field “Hk” can be expressed by the following formula.Hk=3×|λ|×σ/Ms 
According to the above-mentioned formula, under the condition that the saturation magnetization Ms of the magnetic material is fixed, when both the film stress σ and magnetic distortion λ become large, it becomes clear that the anisotropy magnetic field Hk becomes large because of a reverse magnetostrictive effect.
The magnetic head is operated by applying the bias magnetic field of several hundreds Oe in the direction along the film surface of the free ferromagnetism layer. Since the anisotropy magnetic field Hk due to the reverse magnetostrictive effect gives perturbation to the bias magnetic field, it is requested that the anisotropy magnetic field Hk is controlled to be as small as possible, and further it is desirable to make it below 10 Oe in practical. Usually, since the film stress a for the free ferromagnetism layer is below about 300 MPa, it is needed to make the value of the magnetostrictive constant included in a range of ±1.0×10−6 in order to make the anisotropy magnetic field Hk below 10 Oe.
When measuring the magnetostrictive constant of the CoFeB film which is the free ferromagnetism layer of the magnetoresistance effect device described in the above-mentioned reference, it became clear that the absolute value of the magnetostrictive constant is considerably larger than 1.0×10−6. If the absolute value of the magnetostrictive constant of the free ferromagnetism layer is larger than 1.0×10−6, it becomes impossible to cause the units including the magnetoresistance effect device to operate with required performances because the free ferromagnetism layer gives a bad influence on the operation of the units. Then, it is required to reduce the magnetostrictive constant of the free ferromagnetism layer in the magnetoresistance effect device.
In view of the above situations, in order to reduce the magnetostrictive constant of the free ferromagnetism layer of the magnetoresistance effect device, the inventors carried out laminating a NiFe film, which is known to have a small magnetostrictive constant, to the free ferromagnetism layer CoFeB, and tried to measure the magnetostrictive constant of two-layer laminated films “CoFeB/NiFe”. In accordance with the result of the measurement about the two-layer laminated films, its magnetostrictive constant was reduced in comparison with single-layer CoFeB. But an extent of the reduction was not sufficient. Further, in comparison with the single-layer CoFeB, the MR ratio of the two-layer laminated films was fallen down sharply. FIGS. 7 and 8 show the result of measurements about the magnetostrictive constant and MR ratio concerning the laminated films “CoFeB/NiFe” in which the amount of addition of boron “B” is changed. In the change characteristic 61 shown in FIG. 7, when boron B was added, the value of the magnetostrictive constant could fall to about 2.5×10−6. However, it could not be lowered below the value, and further could not be made smaller than the required value 1.0×10−6. In addition, as shown by the change characteristic 62 of FIG. 8, when the amount of addition of boron B was increased, the MR ratio was further fallen down and as a result it was fallen down to about 23% at 20% as the amounts of addition of B.